Sprecher
Beschreibung
Phosphorus is essential in animal nutrition, but poorly available in plant-based feed. Cereals, legumes and oilseeds dominate livestock diets, where phosphorus occurs predominantly as phytic acid (inositol hexakisphosphate). Monogastric animals such as pigs, poultry and salmon have little endogenous phytase activity, leaving most of the phytate bound phosphorus unavailable to them. Diets must therefore be supplemented with mineral phosphate from finite reserves. Both the undigested phytate and the supplemented mineral phosphate raise phosphorus excretion, which drives eutrophication of surface waters and contaminates groundwater.
Phytases address this inefficiency directly. The enzyme hydrolyzes phytic acid to inorganic phosphate that the animal can absorb, lowering both the mineral supplementation required and the phosphorus load released into the environment. Products such as Natuphos from BASF have been approved as feed additives in the EU since the 1990s. For industrial application, the enzyme must withstand the hydrothermal conditions of pelleting, resist proteolysis at gastric pH, and degrade phytate as completely as possible, since partially dephosphorylated intermediates still bind cations and retain phosphorus in an inaccessible form.
Protein engineering has become the principal route to improved enzymes. Structure guided rational design and random mutagenesis coupled to iterative screening have yielded recombinant phytases with enhanced thermostability, acid tolerance and catalytic efficiency. Several enzymes on the market today are engineered variants rather than wild type enzymes.
In my PhD, this work is extended along three lines. A growth-based assay in Bacillus subtilis is being developed that couples secreted phytase activity to growth on phytate as the sole phosphorus source, so that large libraries become directly accessible to screening. Such libraries are to be generated by shuffling characterized wild type sequences into multi parent chimeras, and expression strains are being established in parallel to support secretion of the resulting variants.
